a. Field of the Invention
This invention relates to an electrooptical device using an electrooptical material such as ferroelectric liquid crystal, a method and a circuit for driving the device, and further relates to an electrooptical apparatus employing the electrooptical device.
b. Background Art
Liquid crystal has been widely known as an electrooptical material. Especially, ferroelectric liquid crystal has attracted special interest recently.
A general form of an electrooptical device using the ferroelectric liquid crystal will now be described with reference to FIGS. 2, 3 and 4, which are used for explaining the general idea of the common electrooptical device, but which do not show a specific prior art structure.
The electrooptical device employing the ferroelectric liquid crystal comprises glass plates 2 each having a transparent electrode 3 and alignment layer 4 coated thereon, spacers 6 interposed between the glass plates 2 to space the glass plates 2 from each other and to keep them at a given distance, ferroelectric liquid crystal 5 sealed in a space defined between the glass plates 2, and a polarizer or polarizers 1 placed on either side of the glass plate 2, as illustrated in the FIGURES.
In case the ferroelectric liquid crystal is of a chiral smectic C phase, ferroelectric liquid crystal molecules 7 show spontaneous polarization 8 in a direction perpendicular to longitudinal axes (major axes) of the molecules. The ferroelectric liquid crystal molecules 7 may be aligned in layers 9 which extend in a direction perpendicular to the major surfaces of the glass plates 2 by selecting the alignment layer 4. In the thus-aligned state, the ferroelectric liquid crystal molecules 7 may move substantially along a conical path 10, while keeping a tilt angle .theta. with reference to a normal line 13 of the layer 9.
When an electric field 11 is applied in a direction perpendicular to the major surfaces of the glass plates 2, the liquid crystal molecules 7 may be put into either of two stable positions 12a, 12b which are parallel with the glass plates 2, depending upon a direction of the electric field applied thereto. These two positions are diagrammatically illustrated in FIGS. 4 (a) and (b), respectively, wherein the ferroelectric liquid crystal molecules are shown as being applied with an electric field E (11a) which is directed toward the farther side of the drawing sheet from this side of the sheet and as being applied with an electric field E (11b) which is directed toward this side from the farther side, respectively. Thus, the ferroelectric liquid crystal molecules 7 assume the positions (a) or (b) at a tilting angle of +.theta., depending upon the direction of the electric field applied thereto. This effect may be combined with a birefringent effect or a guest-host effect of the liquid crystal to provide two, i.e., dark and light, states in which light is transmitted in the same direction as the electric field or light is cut out according to the direction of the electric field applied respectively.
It is assumed in the following description, for the sake of convenience, that an ON-state which allows light transmission is developed when a positive voltage sufficient to put the ferroelectric liquid molecules into one of the positions is applied to the molecules, and that an OFF-state which cuts off light is developed when a sufficient negative voltage is applied.
When the thickness of a liquid crystal layer is reduced to 2 .mu.m or so, a threshold effect, such as a memory effect, will be observed. This memory effect may be utilized in an electrooptical device of matrix-arranged electrodes consisting of scanning electrodes and signal electrodes arranged in rows and columns and providing picture elements at intersections of the electrodes. In this device, the scanning electrodes may be selected sequentially, and only the picture elements on the selected electrode may be applied with an electric field whose magnitude is sufficiently larger than a threshold value to set the states of the picture elements, while picture elements on the non-selected electrodes may be applied with an electric field smaller in magnitude than the threshold value to hold the picture elements in the previously set states. Thus, multiplexed driving can be attained.
On the other hand, it has been known that, when an AC electric field, whose frequency is so high that the response based on the spontaneous polarization can not follow the changes of the electric field, is applied to ferroelectric liquid crystal molecules having a negative dielectric anisotropy, a dielectric torque may be produced, which acts to put the liquid crystal molecules 7 parallel with the glass plates 2. This phenomenon is called AC field-stabilization, and it does not depend on a thickness of the liquid crystal layer. This means that, even when the ferroelectric liquid crystal layer has a substantial thickness, it may have a memory effect by the AC field-stabilization effect. This effect may effectively be utilized to enable multiplexed driving of the liquid crystal device which is thick enough to be manufactured easily.
A driving method for the ferroelectric liquid crystal device of the kind is disclosed, for example, in Publication of Japanese Unexamined Patent Application (KOKAI) No. 62-116925. This publication shows a set of driving waveforms as given in FIG. 5. A voltage for putting an electrooptical device into a desired state and an AC high-frequency voltage for holding the state are applied to accomplish the multiplexed driving of the electrooptical device. The method disclosed in this publication further teaches that an initialization signal is applied prior to supplying a selection signal, thereby to put the picture elements once off for every scanning.
Another example of background art is disclosed in National Technical Report Vol. 33, No. 1, Feb. 1987, pp. 44-50. This paper shows driving waveforms as given in FIG. 6. A completely symmetrical AC voltage, in which no bias voltage is applied, is given during a non-selected period. This assures high contrast.
These examples of background art, however, involve some problems, which will be described below.
According to the former first described example of background art, a symmetrical voltage with respect to a zero level is applied for initialization. At this time, a first half of the voltage pulse will forcibly turn on the electrooptical device. Even if an OFF signal is continuously applied to the signal electrode, an ON-state will occur intermittently. This will lower the contrast which is defined by: ##EQU1## In addition, positive and negative bias voltages corresponding to voltages applied to the signal electrodes are superposed in the high-frequency AC voltage during the non-selection period. The bias voltages influence adversely both the ON-state and OFF-state, lowering the contrast. A high-level, high-frequency AC voltage is needed to suppress the lowering of the contrast. This voltage must be completely supplied from the scanning electrode side. This inevitably increases the voltage to be applied to the scanning electrodes.
The second example of background art does not need voltage pulses for the initialization, and it can assure high contrast because of symmetrical high-frequency AC voltage applied during the non-selection period. In fact, however, a voltage of an amplitude twice the amplitude of the symmetrical high-frequency AC voltage applied to the liquid crystal must be applied to all the signal electrodes. A working example of this art shows that a voltage as high as +50V is applied to a device comprising a thin liquid crystal layer of 3.5 .mu.m thickness to drive the same. For this reason, a special high-voltage driving circuit is needed, which makes the circuit bulky and increases the power consumption.